The trend toward miniaturization, integration and automated assembly in the electronics industry is forcing designers to continually increase the component density in integrated circuit manufacturing, interconnection and packaging. Current demand for increasingly complex PWBs has resulted in increasingly stringent requirements for all production steps. To produce high-quality boards at competitive prices means keeping production costs down. This in turn means less consumption of environmentally toxic chemicals, reduced number of manufacturing steps, shorter process times, and a greater need for automation.
The introduction of double-sided, followed by multilayer boards, was achieved by metallization of plated through-holes with electroless copper. For the last 25 years, 98 percent of the PWBs manufactured used this technology. However, electroless deposition of copper requires a potent reducing agent, such as formaldehyde--a reported carcinogen. Most electroless copper solutions contain cyanide and chelating agents, which are difficult to remove from waste streams. Besides the normal drag-out associated with wet processing, "bail-out" (required to maintain solution balance and periodic bath changes) renders waste treatment of electroless copper far more expensive than electroplated copper. Stripping copper from racks and tanks with nitric acid is another environmental and waste treatment concern associated with electroless copper.
In the conventional subtractive plated-through-hole (PTH) process, copper foil is laminated onto an insulating substrate (typically polyimide, epoxy-fiberglass, etc.). Holes are drilled through the copper-clad laminate to allow insertion of components. Then the typical smear and etch-back process uses an alkaline perinanganate solution followed by a hydrofluoric acid solution to remove resin smear and glass fibers from the walls of the holes in preparation for the plating process.
In the conventional process a seed or catalyst, usually a noble metal salt, is then applied to the circuit board. Next, by means of electroless copper deposition about 10-20 microns of copper is deposited on the surfaces of through-hole walls, providing electrical continuity from one side of the panel to the other. Electroless copper deposition is a seven-step process with interval rinses with water that become contaminated with copper sulfate/EDTA/formaldehyde bath components. Following electroless copper deposition, copper is electrodeposited over the entire board surface and sensitized walls of through-holes, usually to a thickness of 0.001 in.
A negative-, or plating-resist, pattern is then applied and registered to both sides of the material. Resist covers all areas of the foil where base copper conductor is not required, and the surplus conductor will subsequently be etched off. The panels are imaged in preparation for the actual circuitry pattern by a conventional photolithographic process. In this process photoresist is applied as a thin film to the substrate and is subsequently exposed in an image-wise fashion through a photomask. The mask (Mylar) is then removed. The areas in the photoresist that are exposed to light are made either soluble or insoluble in a specific solvent termed a developer. In the case of a negative resist, the non-irradiated regions are dissolved leaving a negative image. This is achieved in the development process.
The next plating step is to electrodeposit copper and a thin layer of a suitable etch-resist plating, usually solder or gold. The original plating resist, screen or photoresist, is removed, and the circuit pattern is defined by etching away exposed copper in a suitable etchant (e.g. ammonium persulfate). During this process, 90% of the copper plating is removed by etching, thus producing large volumes of sludge and rinse water.
Recently, the U.S. Environmental Protection Agency's Waste Reduction Innovative Technology Evaluation (WRITE) Program has been established in the printed wiring board manufacturing industry in order to perform technical and economic evaluations of the volumes and/or toxicity of wastes produced from the manufacture, processing and use of materials. Environmental concerns associated with electroless copper metallization, have fostered interest in direct metallization processes. Despite numerous attempts over the last 10 years, conversion to a direct metallization process has not gained widespread acceptance, and only about five percent of PWB manufacturers worldwide have eliminated metallization by electroless copper.
In addition to the environmental concerns about electroless copper metallization, circuit board manufacture using this process can require as many as 15 to 20 steps (including rinses), involving 70 min of processing time. This obviously creates a roadblock for achieving a free-flowing process. Electronics manufacturers have not realized or appreciated the benefits that direct metallization can provide. These include reduced waste treatment/processing costs, lower chemical costs, improved efficiency/reliability, and the elimination of a time-consuming procedure.
Electronically conducting polymers have often been categorized as non-processable and intractable, because of their insolubility in the conducting form. Only recently has it been shown that polymers such as polyaniline can be dissolved using functionalized sulfonic acids. For polypyrrole, this can be achieved by using its derivatives [e.g., poly (3-octylpyrrole)] which are known to be soluble in different solvents, or by treatment in dilute aqueous sodium hypochlorite solutions, ammonia or mono-, di- or tri-substituted amine (co)solvents. Another method of solubilizing polypyrrole is the process of polypyrrole chain deprotonation in basic solutions, which causes a transformation of conducting polypyrrole into a non-conducting polymer of quinoid structure.
The lack of processability of conducting polymer materials, e.g., solution or melt processing, infusability and poor mechanical properties, e.g., ductility, have slowed down their emerging commercial applications. While electrochemical preparation of conducting polymers has been shown to be the most satisfactory process from the viewpoint of fundamental investigations, it is likely to be inappropriate for the large-scale industrial production of bulk quantities of these materials. This is particularly true where large molecular entities, e.g., copolymers or different additives, need to be incorporated into conducting polymer matrices in order to obtain tailored performance characteristics.
In order to compete with more-advanced interconnect systems, such as hybrid circuits and multichip modules (MCMs), future PWBs will have to be designed so that their size and cost advantages can be used to find a wider range of applications. This will require PWBs with increased conductor density. To accomplish this, finer lines and spaces (&lt;5 mils), smaller vias (&lt;12 mils), thinner multilayer boards (&lt;0.032 in), and improved insulation resistance will be necessary. Finer lines and pitch will require high-resolution imaging and precision etching. The presence of plated-through-holes (PTHs 0.062-0.04 in) and vias (&lt;0.10 in) in ever-increasing numbers, will present a challenge in laminating, drilling and metallization.
Consequently, there remains a need for improved direct metallization processes for preparing electronic circuits on non-conducting substrates. It would be desirable to have a direct metallization process that avoids polymer solubility problems, can easily incorporate additives, does not depend upon electroless-copper plating, minimizes hazardous chemicals and copper plating solutions, requires fewer process steps, provides simplified through-hole metallization, and facilitates increased conductor densities.